The present invention relates to surgical devices and more particularly to pumps for surgical implantation in living animals.
At the present time it is known that a pump may be made small enough to be surgically implanted in a laboratory animal. The pump may be filled with a liquid drug or other bioactive liquid. The pump, over a predetermined period of time, expels the liquid from its reservoir into the area into which it has been implanted, for example, the bloodstream or cerebrospinal fluid, or the liquid may be pumped to another area using a tubular catheter.
One such implantable pump is an osmotic pump which may be used for the delivery of drugs at a controlled rate. This osmotic pump is described in the following article: Theeuwes, F. and Yum, S. I. (1976), "Principles of the design and operation of generic osmotic pumps for the delivery of semisolid or liquid formulations", Ann. Biomed. Eng. 4(4), pages 343-353.
Another type of implantable pump is shown in U.S. Pat. No. 3,894,538 entitled "Device For Supplying Medicines", issued July 15, 1975, in which, in one embodiment, gas is slowly produced in one chamber of the pump to force the liquid from a collapsible reservoir. As with the osmotic pump, the rate of discharge of the liquid may not be changed by the experimenter after the device is implanted.
Although implantable pumps have received widespread utilization, they are subject to a number of limitations. In osmotic pumps the rate at which the pump expels its liquid is determined by the osmotic pressure of the fluid surrounding the pump. Since such osmotic pressure is relatively constant, the rate is also relatively constant. While a constant feed rate of liquid is desirable in many applications, in certain other applications and experiments it would be desirable to have the rate both variable and controllable by the experimenter. For example, the osmotic pump may effectively expel its liquid at a constant rate for a one-week or two-week period. There have been indications that longer life pumps may be available which would extend the life to over a month. However, if the rate of pumping of the liquid were under the control of the experimenter, then the liquid may be pumped infrequently, for example, once a week or even once a month, and the supply of liquid may last much longer.
The scientists working with the presently available implantable pumps are limited to those experiments which require that the liquid be pumped at a regular rate and for a relatively short amount of time. They could extend their investigations if they had available pumps which would (a) pump liquid into the subject's body over a longer period of time; (b) pump the liquid at exactly the desired times, i.e., repeated ejections of the liquid are obtained only at the desired moments; (c) pump the liquid in an exact and pre-defined quantity at each pump operation.
In the presently known implantable pumps it is not possible to increase the duration of the pump life by increasing the size of the pump, since it is desirable that the pump which is implanted within the laboratory animal be sufficiently small so as not to interfere with the normal health of the animal. If the pump were made larger, the effect of the experiment would become uncertain since the implantation of the pump itself may injure the animal.
In addition to the implantable pump, it is also possible to introduce drugs into a subject's bloodstream, or other fluids, by implanting slow-release materials such as drug pellets. One type of drug pellet contains micro-capsules which dissolve at different rates and release the drug over a selected time period. However, in drug pellets the rate of release is generally relatively rapid--a matter of days--and once the pellet has been implanted the rate of release is not controllable. The release rate may depend upon blood circulation and may vary, depending upon the individual subject. Such a varying and unknown rate is generally undesirable in an experimental situation. In addition, the release may be blocked by the body defense means, such as tissue growth. Such slow-release materials often fail to be available at the desired body location and at the desired concentration.
It will be understod that, although the discussion herein is in terms of "laboratory animal" or "subject", the invention is applicable to wherever a small externally activated pump may be utilized. For example, such a pump may, in the future, be approved for surgical implantation in humans as a means of ejecting a measured amount of drugs into the bloodstream.